1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus. More particularly, the present invention relates to an image forming apparatus configured to minimize by-product material to be inevitably generated in a fixing unit for fixing a toner image on a paper sheet by pressing under heating.
2. Description of Related Art
Conventionally, in an image forming apparatus such as a copier, a printer, a fax machine, or a multifunction machine that combines their functions, a toner image transferred on a recording paper sheet is fixed by a fixing unit and thus an image is formed on the sheet. Specifically, the fixing unit carries out a fixing process to cause the sheet to pass through a fixing nip formed by a pair of fixing members placed in pressure contact with each other and press the sheet under heating at the time when the sheet passes through the fixing nip. The fixing members include a cylindrical roller, an endless belt member, and others.
Relating to the fixing unit, it is recently known that ultrafine particles (UFP) are generated in the fixing process. To be concrete, UFP are conceived to be generated from silicone material forming the fixing members, toner melted under heating, and others in the fixing process. To address this disadvantage, for example, JP-A-2012-32663 discloses a technique to collect UFP generated near a fixing unit. In JP-A-2012-32663, a filter impregnated with silicone oil is provided in an exhaust duct of the fixing unit. According to JP-A-2012-32663, it is expected that this filter impregnated with silicone oil can efficiently collect the generated UFP from the fixing unit.
Furthermore, JP-A-2008-40310 discloses an image forming apparatus in which one of a pair of fixing members of a fixing unit is provided with a cleaning member for cleaning the surface of the one fixing member placed in contact with a toner image bearing surface of a recording sheet. This cleaning member is switched by a changeover means between a pressure-contact position in which the cleaning member is placed in pressure contact with the fixing member and a separated position in which the cleaning member is separated from the fixing member. In a case where a large amount of toner adheres or sticks to the fixing member, the toner adhering to the fixing member is removed at the pressure-contact position. This is to prevent the toner adhering to the fixing member from transferring to a recording sheet passing through a fixing nip, thereby preventing generation of noticeable stains or smudges on the sheet. By cleaning using such a cleaning member, the UFP to be generated from the fixing unit is suppressed to a certain extent. This is because the toner adhering to the fixing member is removed and thus the toner to be heated in the fixing process is reduced.
Meanwhile, in order to collect the UFP by the aforementioned conventional method in JP-A-2012-32663, special components for collecting UFP such as a fan and a filter are required. This would cause a problem with complexity of the image forming apparatus due to those components for collecting UFP. Further, completely collecting all the generated UFP is difficult. It is thus preferred to reduce the amount of UFP to be generated.
The technique in JP-A-2008-40310 is directed to clean the fixing member to prevent the generation of noticeable stains on the recording sheet. In other words, the fixing member may be adhered with toner of such a degree that will be unnoticeable when the toner transfers to the recording sheet. The amount of UFP to be generated from the fixing unit differs according to conditions of jobs to be executed. Thus, if even the toner of an amount to the extent that will be unnoticeable when the toner transfers to the recording sheet adhere to the fixing member, this would cause a problem with the generation of a large amount of UFP in executing a job that is liable to cause an increase in generation amount of UFP.
Moreover, the generation amount of UFP can be reduced by frequently cleaning the fixing member. However, such frequent cleaning may deteriorate the productivity of image formation in the image forming apparatus configured to disable the image formation during cleaning of the fixing member. Cleaning also would cause a certain degree of load on the fixing member. Consequently, highly-frequent cleaning may shorten the life of the fixing unit.
The present invention has been made to solve the above conventional problems and has a purpose to provide an image forming apparatus configured to suppress a generation amount of UFP and also prevent deterioration in productivity of image formation and life of a fixing unit.